This invention relates to improved coating materials for particulate solids, and especially for fillers which are useful in solid rocket propellants. More particularly, it relates to coating materials which are neutral and which enhance the bond between the powdered solids and binder material which are propellant components.
When a propellant is subjected to mechanical stress, failure of the matrix will occur when the stresses in the binder phase reach a magnitude comparable to the elastic modulus of the unfilled matrix. Owing to stress concentrations, the matrix in the immediate vicinity of the filler particles will be the first to fail, causing the formation of tiny voids. If the load on the specimen is increased further, eventually a state is reached where the thin membrane of the matrix separating the particle from the void breaks, causing a sudden and complete withdrawal of the matrix from the solid. This situation is usually referred to as dewetting or blanching. In a "dewetted" propellant, the filler has lost its reinforcing effect, and a structurally very weak material results in which the entire load is borne by the matrix. It is well known that propellants which dewet have poor strain cycling ability.
If the filler particle is attached to the matrix by primary chemical bonds (e.g. an epoxy resin in a polyurethane binder), dewetting does not occur and, therefore, the filler does not lose its reinforcement. Consequently, the composite will reach higher stresses and elongations before failure. Fillers which per se do not contain functional groups to form this linkage to the binder can be converted to reinforcing fillers by enveloping them with a shell of an appropriate material.
It was previously believed that an effective coating required residual amino or OH groups in order to form the required primary bond with the matrix (e.g. a polyurethane binder). Therefore, the early coatings were usually either polyurethanes or reaction products of amines with epoxides. To produce such coatings on particles is relatively cumbersome due to the long reaction times required. Furthermore, in the case of epoxy-amine coatings, the basic nitrogen remaining can impose serious problems with some other propellant ingredients. Particularly for the intended purpose of these coatings, basicity is not tolerable since it is imcompatible with most high energy plasticizers, which are an important part of certain high energy, high impulse solid rocket propellants. Additionally, the catalyst triphenylbismuth (TPB), which is used in virtually all nitro- or nitratoester plasticized high energy propellants, does not function in the presence of basic impurities.
To overcome the problem, certain polyurea coatings were developed. However, these coatings were only partially successful; while the reaction between the amine and an isocyanate was practically instantaneous, the resulting compound still contained basic impurities. For example, a tetraethylenepentamine/acrylonitrile coating was developed that gave a significant improvement of the mechanical properties of bis(fluorodinitroethylformal) ("FEFO") plasticized polyethylene glycol propellants. Although the coating contained considerable basic impurities, basicity was not a problem with such propellants because of the extreme base sensitivity of FEFO, which immediately neutralizes basic impurities, albeit with decomposition. However, such "self-neutralization" is absent in nitrate ester systems, so that when the coating was used with nitroglycerin ("NG") plasticized propellants, cure interference was encountered. Accordingly, such coatings could not be used for nitrate-plasticized propellants.
Therefore, the problem remained to develop a coating that reacted quickly to coat powdered propellant solids while eliminating basic impurities.